The invention relates to an optical system imaging an object at infinity over an intermediate image in an image plane, the optical system having a front group and a relay optical system, which in each case has several lenses and between which the intermediate image lies.
In optical systems, which image radiant heat, stability of the field of view of an object is required over a certain temperature range. Usually this is achieved by manual or by automated refocusing by a motor. With an appropriate optomechanical design, the variation of the focus due to temperature can be minimized to such an extent that it remains within the depth of field and thus remains imperceptible, practically speaking. Refocusing is then no longer necessary, but the cost of the moving components is definitely greater than in uncompensated systems. This kind of optomechanical design of optical systems with manual or automated (motor-driven) refocusing is referred to hereinafter as “active dethermalization.”
Dethermalization of focus can be realized, however, even without mechanically operated components by selectively tuning optical components and mechanical supports. This will be referred to hereinafter as passive dethermalization
Thus, an optical system of this class is known (DE 19807093 A1), with which, for a single focal length or a single field of view, extensive temperature invariability of the image position can be achieved.
Especially in the case of passive dethermalization for the infrared spectral range, the problems with the generic optical system increase due to the poorer selection of available components that can be used. For this reason a complete passive dethermalization frequently cannot be performed. In this case, a certain residual error in the image position is then tolerated.